A TEM STUDY OF OXIDE LAYERS FORMED DURING DECARBURIZATION ANNEALING OF ELECTRICAL STEEL

Abstract

It is very important to know the morphology and chemical properties of subscales of oxide layers on surface for controlling and understanding high temperature oxidation in electrical steel. In present work, the oxide layers were investigated by various methods of transmission electron microscopy (TEM) such as scanning transmission electron microscopy (STEM), nanobeam electron diffraction (NBD), energy dispersive X-ray spectrometry (EDS), and electron energy loss spectrometry (EELS). The high-angle annular dark field (HAADF) of STEM could be a useful analysis technique to study the morphology of the oxide layers. The main oxides formed in the subscales during the decarburization annealing were fayalite, iron oxides, and silica, which were identified by EDS, NBD and EELS. The crystalline fayalites were found both in the surface region within several tens nanometers and in the region within a micrometer surrounding silica, and the atomic configuration in the unit cell of the fayalite was presented. Amorphous silica was formed both in the upper region of the subscales with a spherical shape and in the interface between the spherical silica and the iron matrix with a lamellar shape. TEM could be useful technique to characterize morphologies, microstructures and elemental compositions of oxides, and to understand the oxidation mechanism for the manufacture of the high quality electrical steel.